1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a low-loss power semiconductor device.
2. Description of the Related Art
Recently, low-loss semiconductor devices have been developed. One type is an Insulated Gate Bipolar Transistor (IGBT), invented during the middle 1980s. The ON-voltage of the first generation IGBT at the early period of the development was 4 V, the ON-voltage of the second generation IGBT developed in 1988 was 3 V, and the ON-voltage of the third generation IGBT developed in 1991 was 2.4 V. Thus, as can be seen above, the power loss of IGBTs have been rapidly reduced. Further, a lower-loss device has been developed to provide the basis for developing a fourth generation IGBT having an ON-voltage of 1.5 V. The following is an outline detailing a process for developing a low-loss IGBT device.
In FIG. 1, a cross-sectional view of a conventional IGBT structure is shown. A n- layer is formed on a substrate forming a p+ layer. The n- layer secures a withstanding voltage by extending a depletion layer and relaxing an electric field. A gate electrode is formed at the surface of the n- layer via an oxide film. A p layer is formed in the n- layer at both terminals of a gate electrode, and an n+ layer is formed in the p layer. The n+ layer and the p layer are short-circuited by an emitter electrode. A collector electrode is connected to the n+ layer by ohmic contact. The semiconductor device operates as follows. At first, positive voltage is applied to the collector and the gate, in contradistinction to voltage applied to the emitter. If the voltage applied to the gate electrode exceeds a threshold value, the inversion occurs in the p layer under the gate electrode and electrons flow into the n- layer. Then, by the electrons, the forward bias is applied to a pn junction formed by the n- layer and the p+ layer, and if the voltage applied to the pn junction exceeds diffusion potential, holes are injected from the p+ layer to the n- layer. Then, electrical conduction of the n- layer having high resistance is modulated by the injected holes, and the high resistance is decreased.
Since the amount of hole current is determined by the amount of electron current from MOSFET formed at the device surface as mentioned above, lowering the loss was conventionally realized by decreasing a channel resistance of MOSFET and increasing the electron current. The below-mentioned methods have been adopted as the method of decreasing the channel resistance, that is, (1) decrease of the channel width per unit area by reducing the length of the p layer and the n+ layer of the emitter by use of an advanced fine processing technique; (2) decrease of the channel width by reducing the thickness of the n+ layer and the p layer; and (3) decrease of the thickness of the oxide film at the gate electrode.
The length of a gate should be kept more than a definite value since the region in which electrons flow is narrowed by extension of the depletion layer from the p layer. Since the ratio of the length of a gate to that of an emitter forming region has reached about (6:1) already in the third generation IGBT, the channel width per unit area is not increased so much by making the emitter region finer and the decrease of the thickness of a p layer is limited since the withstanding voltage has to be secured. Further, the thickness of a gate oxide film can not be decreased more since the gate voltage is fixed. Therefore, the method of increasing electron current by decreasing the channel resistance of MOSFET has reached its limit.
In view of the foregoing discussion, it is clear that the current does not flow in the IGBT until the pn junction is biased forward. The forward bias voltage is about 1 V in a conventional IGBT made with silicon, about 40% of the ON-voltage of 2.4 V for the third generation IGBT, and about 70% of the ON-voltage of 1.5 V for the fourth generation IGBT. The forward bias voltage applied to the pn junction can be decreased by narrowing a band-gap of semiconductor. For example, use of germanium instead of silicon has proven to be effective. However, in a semiconductor having a narrow band-gap, the loss becomes larger since the leak current increases in the high temperature state.